Vacuum flask



FIG. 1 is a perspective view of a vacuum flask showing my new design thereof;

FIG. 2 is a front elevational view and the rear side elevational view being a mirror image thereof;

FIG. 3 is a left elevational view and the right side elevational view being a mirror image thereof;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof. 

The ornamental design for vacuum flask, as shown and described. 